A practical way to double the bit carrying capacity of an existing unidirectional fiber optic communication link is by the use of optical circulators. An optical circulator is a passive, non-reciprocal device which permits full duplex communication on a single fiber optic link. Thus, a typical fiber optic communication link operating on two fibers can be quickly and economically converted to a bi-directional, single fiber communication link by installing an optical circulator at each end of the link.
One of the major advantages of optical circulators over more traditional 3 dB couplers is that the loss penalty is much lower. Using a 3 dB coupler at each end of a fiber link, there is an insertion loss of at least 6 dB. For connections which operate near their detection limits, this additional 6 dB loss could make bi-directional communication impracticable.
In a real optical circulator insertion loss and cross-talk are two important considerations. Insertion loss is the difference in power between light launched into the optical circulator and the power that exits the device. Insertion loss is largely due to absorption of light and to imperfect polarization separation.
Prior art optical circulators are described in U.S. Pat. No. 4,650,289, issued to Kuwahara; U.S. Pat. No. 4,464,022, issued to Emkey; and in U.S. Pat. No. 4,859,014, issued to Schmitt et at. However, insertion loss and/or cross-talk in optical circulators made as described in these references are unacceptably high for many communications applications. Therefore, a need exists for an optical circulator having lower insertion loss and cross-talk than that found in present optical circulators.
One factor that contributes to lower insertion loss and cross-talk in the optical circulator of the present invention than in prior optical circulators is the use of birefringent crystals instead of traditional polarization spitting cubes results in much more complete polarization of incident, randomly polarized light.
A more recent prior art optical circulator that uses birefringent crystals in contact with other elements such as polarization rotators, is U.S. Pat. No. 5,204,771 issued Apr. 20, 1993 in the name of Koga. Although this invention appears to perform its intended function adequately, precise matching of optical components such as waveplates is required; this precision matching is found to be difficult and thus costly. This requirement of matching reciprocal rotators is obviated in the present invention which overcomes this problem by utilizing a centrally disposed split pair of birefringent crystal plates.
It is therefore an object of the invention, to overcome many of the limitations of known prior art devices.